The Botany of Iceland - 01.12.1914, Síða 79

PHYSICAL GEOGRAPHY
263
land1 and in places where the water, for some reason or other,
cannot drain off. Here the soil is usnally much thinner than
in the home-fields but nevertheless it has a very great power of
absorbtion during the process of surface-evaporation; the greater
part of these knolls consists of móhella-clay. On an entirely bare
claý-flat no knolls are found, and the separate cakes in the „rude-
mark” rise but slightly in the middle during spring, but not until
they are plant-covered and clad with greensward do they bulge and
retain their convexity. On a closer investigation it is probable that
several transitional forms will be found between “rudemarks” and
knolly flats, but investigators have not as yet made this matter a
subjeyt for study. Where similar plant-formations (heather and
“móar”) occur upon slopes or mountain sides upon which tlie water
easily finds outlets, knolfy flats are absent, and “rudemarks” are
never found in such localities. Upon mountain-sides small eminences
of various kinds may sometimes be observed which are due to under-
lj'ing stones or ridged mud-flows, etc., but never “thufur” proper.
Nowhere have I seeu any indications of “rudemarks” or “thufur-
marks” being at all connected with mud-flow phenomena.
On the plateau peculiar knolls of usually large size are found
which the inhabitants of the different districts call “rústir,” “haugar”
or “dys.” These knolls are, as a rule, of irregular, oblong form,
and are bare of vegetation on the top, where tliey consist only of
humus and clay. It can be seen thal the ground has bulged and the
knolls are cracked at the top. In some places there are evident
proofs of this being the case: bands of the original soil are seen to
lie upon the top of the knoll while the clay and gravel within have
poured out of the cracks bel\veen the bands. In tbe neighbourhood
of Ulfsvatn on Tvidægra, al a height of 460 metres above sea
level, I investigated such knolls in 1898; they were 1—IVa metres
in height, 15—20 metres in length and 8—10 metres in breadth.
The sides were covered with heather, but Cyperaceæ grew in the
intervening spaces. Similar knolls occur in several olher places in
the interior of Iceland, but only in one more place did I observe
1 Dr. H. Jónsson deseribes a heather-eovered “rudemark” where the cracks
between the cakes were covered witli Grimmia lujpnoides. but the cakes themselves
with Calluna and Empetrnm. This peculiar vegetation — the brownish heather-
vegetation divided into numerous polygons by the greyish moss-bands — occurred
only upon level ground (Botanisk Tidsskrift, XXVII, 1905, pp. 43 and 44).